There is lots of speculation in that thread, but Bruce has a pretty clear answer:
AFAIK, <foo src=""> tells the browser to get something and insert it here - eg <img src="">, "<script src="">. Stylesheets aren't 'inserted', they are related to the current doc, but typically style more than 1 page. <style></style> declares a block of rules for this page only
I sort of get that. The location in the document matters with src, but not with <link> — that relates to the entire document instead. I guess the crack in that reasoning is that the order of stylesheets does matter for order-specificity, but I take the point.
The W3C chimed to confirm that logic:
(2/2) and the original <script> element in HTML 3.2 didn't have a src attribute. the REC for HTML 3.2 mentions <link> as a way to link to scripts, but it doesn't define any keywords for rel=.